Effective illumination of liquid crystal displays (LCDs) to permit clear, crisp viewing under a variety of ambient lighting conditions has been and continues to be an area of intense scientific, engineering, and business interest. For example, new markets have developed for lightweight laptop computers and other devices demanding high visibility LCDs used in applications where ambient illumination varies from zero, or total darkness, to bright sunlight. Backlighting with supertwist crystal displays has been developed to satisfy these illumination extremes. With respect to such technology, applicant respectfully identifies U.S. Pat. No. 4,655,553 issued to applicant's assignee. The disclosure of that patent is incorporated by reference herein.
The problem which the present invention solves relates to the need to provide nighttime instrument visibility, for example, in the cockpit of a military aircraft, where night vision goggles, particularly of the GEN III type, are being worn by some, but not necessarily all crew members. Both the users of such goggles and those directly viewing cockpit instruments employing liquid crystal displays should be able to clearly see information presented by such displays when they are backlit or trans-illuminated for use in a night vision cockpit environment. Similar night vision applications for the present invention exist in other types of vehicles, such as, but not limited to military tanks and ships.
Color and intensity of illumination of instruments, especially those employing liquid crystal displays (LCDs) which require ambient, integral, or backlighting to be visible has been determined for particular applications, including those within an aircraft. The present invention addresses the problem of spectral content of the illumination in a display system where both direct viewing and viewing through GEN III night vision goggles is required in a night vision environment.
In situations where night vision goggles are being used by military flight crews to enhance terrain visibility outside the aircraft in minimal, natural, nighttime illumination conditions, the visibility of illuminated LCD instrumentation in the cockpit becomes a serious problem. The illuminated or trans-illuminated LCD displays obtain light typically from incandescent bulbs. The infrared radiation emitted by incandescent bulbs is communicated through the rear of the LCD to the observer in front. An observer wearing night vision goggles will see only a green blur with no visible LCD characters if a green type A filter specified in MIL-L-85762 is used in front of the LCD between it and the observer to remove infrared radiation on its way from the LCD to the observer. It should be obvious that use of any filter in front of the LCD diminishes the light transmitted through the crystals and available to the observer. In addition, a frontal filter eliminates the benefit of ambient incident light reaching and being reflected by the liquid crystal display. If the incandescent backlighting is increased to overcome the attenuation caused by frontal filtering, the observer wearing night vision goggles sees an even greater blur as a result of the blooming effect the infrared radiation has on the goggles. If the incandescent illumination is reduced to a threshold level below which blooming will not occur, the crystal illumination level is inadequate for viewing with the naked eye.
The problem of appropriately illuminating liquid crystal displays which are employed as readouts for various input devices may be more pronounced depending on the construction of the LCD itself. Thus a multilayer LCD may have a two per-cent light transmission factor. With little or no ambient light for reflected illumination, a multilayer LCD will require a substantial level of backlighting with a corresponding high level of infrared output being transmitted through to the observer. The observer using infrared sensitive night vision goggles or other night vision equipment will, therefore, be blinded or have night vision impairment to an unacceptable degree. The offending infrared must be removed or eliminated to a degree that it does not interfere with the use of the night vision equipment.